Vacuum tube oscillator



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Patented June 11, 1935 UNITED STATES VACUUM TUBE oscILLA'roR Ralph I. Cohn, Oceanport, N. J.; now by judicial change of name Ralph Cole Application February 11,

1932, serial No. 592,287

4 Claims. (Cl. Z50-36) (Granted `under the act of March 3, 1883, as amended April ,30, 1,928; 370 0. G. 757) The invention described herein maybe manufactured and used by or for the Government for governmental purposes, without the payment-to me of any royalty thereon.

This invention relates to vacuum tube oscillators and proposes a system for varying or controlling the frequency of modulation of an oscillatory circuit which does not require an external input voltage.

`An object of the invention is to provide for varying or controlling the frequency of modulationby varying the impedance of the gridcircuit, or by varying the impedance of the plate circuit.

Other objects of the invention are to employ a step by step, or continuously variable modulated oscillator to excite a `power amplifier; also for obtaining step by step audio modulation.

Another object of the invention is to employ such a system as a combination transmitter-receiver, that is to say the circuit functions asa self-modulated oscillator when transmitting, and asa super-regenerative receiver when receiving. Other objects will appear as the description proceeds. i

' Referring to the drawings: Y

Fig. 1 shows a fundamental circuit forl-carryv y ing out the principles of the invention; f I

Fig. 2 shows graphically the"audio`fre`quency response to impedance variations of the grid circuit;

1 Fig. 3 is a similar view showing audio frequency response to impedance 4variations of the plate circuit;` A

Fig.` 4 is a circuit arrangement embodying, .means/for changing the plate impedance to vary the audio frequency; l l

"Figr shows graphically continuous audio vfrequency `response to impedance variations of th platefcircuit with head-phones removed; 1.

Figs. 6 and 7 show arrangements for obtaining f different frequencies of modulation by switching action withhead-phoneremoved;

Fig. 8 shows a circuit arrangement for lobtaining an audio voltage for external modulation;

Fig. 9` shows a circuit arrangement adapted to provide a continuously variable audio oscillator useful for any purpose requiring such! -frequencies;` and l l Figs. 10 to 14 are a series of curves to illustrate Jgraphically the automatic step by step Ysystem of control of audio frequency in accordance with the present invention. Y l

As will be seen by reference to Fig. 1, an oscillatory circuit is illustrated, which is generally similar to Well known `detector arrangements,

. frequency oscillator.

but provision is made for various combinations of voltages and 4changes in circuit constants to permit the generation of audio and radio voltages in accordance with the principles of the invention. The circuit shownby way of example is a 5` regenerative feedback system of well known design comprising a vscreen grid or four electrode tube I, having a. filament or cathode 2, control grid 3, plateor anode 4 and anode screen grid 5. The filament is energized or heated from a source of direct currentenergy 5 through a variable resistor l. The tuned grid or oscillating circuit 'is conventional, comprising a coil 8, va variable tuning condenser 9, and having the usual shunt connected grid condenser I0 and 1 grid leak II. The plate-lament circuit comprises aninductance ortickler coil I2, a translating device such as telephone receiver I3, a plate battery shown as a source ofdirect current energy I4 and a tuned tank circuit I5, said tank circuit comprising a choke coil I6, tuned by variablexcondenser Il in shunt therewith., The; screen grid 5 is maintained at a positive potential, the vvalue of which may be varied, and is shown connected to a Apositive electrode of plate battery I4 :through a `variable or adjustable resistance I8. Antenna A is showncoupled to the grid circuit through a variable condenser I9.

l Experiment has shown thatwhen such a circuit is operated inaccordance with the present invention,` it is possible to `secure an audio voltage or Wave of varying pitch, which is available either for modulating another tube, or to modulate the radio frequency developed in the said circuit, or in other words, to obtain vva self-modulated radio The mode of operation is as` follows: With the filament voltage adjusted to the proper value for thetube used, for example two volts, and this plate voltage approximately 170 volts, the screen 40 grid voltage is adJusted by means Yof variable resistor I8 (Fig. 1) vuntil an audio note. is heard in the headphone I3.

Nowif the condenser 9 be rotated, a change inthe audio frequency will result or in, other Words, as the impedance is` varied, a change both of thexr'adio and audio frequency is effected. VWith the circuit shown in Fig. 1, this change will not occur gradually but in sudden jumps as graphically shown in Fig. 2. It will be noted that as the impedance is increased, `5() the frequency of the audio wave is decreased.` Hence it was possible, by simply varying condenser 9 to obtain various audio notes on down the musical scale. This action may be explained as follows: A `change in the frequency tov which the grid circuit is tuned, changes the impedance of that circuit. Now in an oscillating circuit comprised of inductance 8, condenser 9, grid leak Il and grid condenser I0, a change in the impedance will, providing the grid leak and condenser combination is correct for grid modulation, change the period of'blocking of said combination; or in other words, vary the audio frequency which will appear in the plate circuit of that tube. Thus,

in reality, we have a radio frequency oscillator" modulated by the use of the grid leak and condenser arrangement as shown. More will be said later regarding the use of this circuit as aself.-

modulated oscillator. It is apparent that for this` purpose the emitted radio frequency must be held constant. The means for so doing will now ber discussed.

The control of the frequency of modulation is possible by varying the impedance of the lplate circuit, inasmuch as a change in impedance in the plate is reflected back in the grid. Thus in Fig. 1, by Varying the condenser Il, the audio frequency may be varied, keeping the radio frequency constant. The same definitev steps are obtained, as graphically illustrated in Fig. 3. u

It having been establishedl that a change in the plate impedance will Vary the audio frequency, divers means can be employed with this idea in mind. Fig. 4 represents this variation secured by i means of resistance 20. Up to this point, the audio frequency variations secured have always been in sudden jumps or steps. Now it has also been noticed that if the headphones be removed from the circuit and the plateV impedance varied, a continuous change in audio frequency will result. This is .illustrated in Fig. 5. What is therefore needed in the plate circuit is sudden changes in impedance to bring about sudden changes in modulation, inasmuch as it has been established that a change in plate impedance changes the audio frequency. Now the reason why sudden jumps were secured with the headphones in the circuit is that actually sudden changes in plate impedance take place when the plate impedance is va-y ried either by in Fig. 4, or condenser I] vin Fig. 1. That is to say, if the plate impedance be lowered, the impedance of headphones might rise suufciently to permit a constant total impedance over certain limits. Now a change in loadimpedance will change the plate voltage, and hence a somewhat lower family of curves will result. VBy a suitable combination of` initial plate voltage, thereiect of a step-by-step change Ain total im pedance is secured. i

Figs. 6 and 7 show how different frequencies of modulation could be obtained by switching Vac tion without the headphones in the circuit. In

the modification shown in Fig. 6, a series of con-A densers of different capacity values are employed.

as shown at 2|, which may be inserted in the plate circuit selectively. In the modification shown in Fig. 7 a series of resistances of Varying values as 22 may be selectively inserted. in the plate circuit. Fig. 8 shows how an 'audio voltage coul-d be secured for external modulation by simply coupling across the headphones or choke coil with the same frequencycharacteristic.

Fig. 9 shows acircuit arrangement adapted for use as a continuous variable audio oscillator for any purpose requiring said frequencies. By 'inserting a pair 4of headphones in this circuit, which would be equivalent to Fig. 1, a step-by-step control could be obtained.

It has been mentioned that a self-modulatedA lradio frequency oscillator could be secured with these same circuits, the emitted frequency being determined by coil B and condenser 9. Either capacity or inductive coupling could be employed to feed either a power amplier or antenna system. Tests were made by feeding a single wire antenna thru a 35 m. m. f. condenser, using a frequency of approximately 4000 k. c. The modulation was varied by changing the plate impedance by means of the condenser i1 in Fig. 1 and Vresistance 20 in Fig. 4, both giving very satis` factory step-by-step modulation. The diaphragms'were then removed from the phones and the experiments continued with the same results as before. Next the headphones were removed from the circuit and a transformer primary substituted as shown in Fig. 9. A continuous variation in modulation frequency could be obtained in the receiver headphones.

The theory of automatic step-by-step system of control of audio frequency is elaborated as follows:

Fig. 10 shows the effect upon the d. c. voltage at the plate of the tube when the variable impedance in the plate circuit is varied. It can be seen that if the impedance be changed from Z1 to Z2 by adjusting the variable impedance, the voltage is reduced from Epl to Epi. The condition is satisfied only on the upper bend of the Ip-E curve, said condition being satisfied during periods of saturation existing as part of the superregenerative cycle. This reduction in plate voltage causes the plate impedance of the tube to decrease as is illustrated in Fig.V 1l. Since it has been shown that a change in plate imepedance causes a change in audio frequency, the total impedance having been changed from Zl-i-Rpl to Zz-i-RDZ, would cause a continuous change in the modulating frequency with a continuous change in Z1 to Z2.A This is illustrated in Fig. 12.

It has been shown experimentally that if a pair of headphones or other suitable choke with the same frequency characteristic be inserted in the circuit, a step-by-step change in total impedance can be secured. A possible explanation is as follows: The frequency characteristic of a pair of phones is illustrated in Fig. 13 wherein it can be seen that if the audio frequency flowing in the circuit be increased, an increase in impedance results up to a critical value Zphl, thence displaying a reversed characteristic until the impedance isdecreased in value to Zphz whence the curve once again reverses itself. Now, if over certain limits of the variable impedance, the total circuit'ime` pedance can be made to remain constant, and then suddenly change, a step-by-step control of the modulating frequency willresult. The total circuit impedance when the variable part of it is equal to Z1 is Zi-i-Zpi-l-Zph: When the variable impedance has been increased to Z2,A the plate impedance Rpg is reduced, but not as 'much as Z2 has been increased, and a reduction inaudio frequency would tend to occur. This is delayed, however, by the fact that the impedance of the phones first tends to decrease as the frequency of modulation is reduced say from f1 to f2, then increases as the frequency continues to decrease to f3 and again decreases as indicated, thus Vin effect securing a form of choking action; that is to say, the impedance characteristic of the phones is such that the total circuit impedance of the tube circuit remains stationary over certain limits, notwithstanding the fact that the variable load may change.

Thus, bythe insertion of the phones, the audio frequency-can be held constant as long as ythe total circuit impedance comprising the plate impedance, variable load impedance, and phone impedance does not change, regardless of whether one or more of the variables change. When finally, the variable impedance has been increased to such an extent that the total circuit impedance is changed, a change in the audio frequency is secured. This occurs in a sudden jump owing to the characteristic impedance of the phones as is shown. Fig. 14 shows a typical curve of audio frequency vs. variable impedance.

I claim:

1. An oscillating modulating system, comprising anelectron tube; grid and anode circuits operatively associated therewith and adapted to cause said tube to function as an oscillator primary tuning means for the grid circuit; means in the gridcircuit to cause a gridblocking action at an audio rate; means in the anode circuit acting independently of said primary tuning means comprising a uniformly and positively controllable variable impedance element, and a load impedance external thereto which responds to the control of said element and automatically increases in absolute value with increase in frequency between predetermined limits, to cause step-by-step increase in the total anode impedance, and reacting on the grid circuit to effect a corresponding step-by-step control of the grid blocking action.

2. In a system of the character described, comprising an electron tube having grid and anode elements; grid and anode circuits operative with said elements and adapted to cause the tube to function as an oscillator; means in the grid circuit to cause a grid blocking action at audio frequency; and means in the anode circuit comprising a series of resistances to eiect step-by-step control of the audio frequency rate of said blocking action.

3. In a system of the character described, comprising an electron tube having grid and anode elements; grid and anode circuits operative with said elements and adapted to cause the tube to function as an oscillator; means in the grid circuit comprising a condenser and grid leak to cause a modulating action at audio frequency; and variable impedance means in the anode circuit comprising a. series of resistances and including a translating device to effect automatic step-by-step control of the audio modulation of the system.

4. `In a system of the character described, comprising an electron tube having control grid and anode elements; circuits operative with said elements and including a regenerative circuit system adjusted to cause the tube to function as an oscillator primary tuning means for the grid circuit; means in the control grid circuit to cause a grid blocking action at an audio rate; means for uniformly varying the impedance of the anode circuit comprising a positively controllable reactance acting independently of said primary tuning means; and a supplemental impedance in said anode circuit which changes automatically under control of said reactance and step-by-step with changes in the total anode impedance, thereby reacting on the grid circuit to effect step-by-step control of the rate of grid blocking, and independently of an external signal voltage on said grid circuit.

RALPH I. COHN, Now by judicial change of name Ralph I. Cole. 

